This invention relates to improved speed controlling apparatus for a mill drive.
The invention will be described in relation to a steel rolling mill but it will be apparent that it could be used in drives for other mills and as a speed controller for other apparatus where similar conditions occur.
Speed control apparatus is well known for controlling mill speed. Basically such control apparatus has a sensor which detects actual mill speed and compares the actual speed with a reference speed or desired speed to derive an error signal. The error signal is used to control the power to the motors of the mill drive to reduce the error signal and cause the actual speed to approach the reference speed.
Speed control apparatus has a time constant or speed of response selected by the designers of the apparatus. The actual speed should be maintained as closely as possible to the reference speed and when a disturbance causes the actual mill speed to depart from its desired speed, it should be returned as quickly as possible to the desired speed. If, however, the speed of response is too rapid, then hunting or other instabilities may be introduced which might cause damage or excessive wear of the drive.
Selection of a suitable time constant is usually a compromise. The speed of response must not be so fast that minor changes in the actual speed cause hunting, but it must be fast enough to cause large deviations of the actual speed from the reference to return to the reference speed as soon as possible. In this connection it is known to use two response speeds: one when the error signal is below a certain level and the other when the error signal exceeds this certain level. While a two level system such as this is able to handle larger changes, it has a disruptive effect at the point of changeover.
A steel rolling mill, and perhaps other large mills, have conditions which make accurate speed control difficult. These mills are subject to fairly abrupt changes in load, for example, when a metal bar or ingot enters the bite of the rolls. Also, the mill drives are quite massive and have considerable inertia. The prior art speed control systems do not respond rapidly enough to abrupt changes in load torque to bring the mill back to speed in sufficient time for efficient mill operation.